25mm would be fine imo for XC, though I wouldn't want to go any narrower. If I was spending decent money on a new pair of wheels these days though I'd probably be looking for closer to 30mm. My current XC bike has 25mm rims, though the bike is nearly 10 years old - my enduro bike has 30mm. Depends a little bit how hard you plan to be riding - if you're fairly gentle then 25mm would definitely be fine.

Depends on the width tyre you are planning to use. 30mm can be a bit two wide for some 2.25" rear tyres and round the profile out too much. If your frame can run 2.35 or 2.4, then get 30mm.

Surely nobody is riding 2.25" any more?!!!

Ha ha true, unless it's an older frame, particularly a hardtail.

I ride specialized 2.35mm tires down to 15 psi on 25mm rims with no issues. 30 may be better but 25 works fine.

I run from 2.25 to 2.4 with 28mm inner width rims.

If you’re upgrading to one or the other, I’ve felt there is much more stability in 30mm and 2.4”.

Thanks. I agree. I initially moved from too small 21mm inner width carbon rims to inexpensive aluminum 28mm rims (with DT Swiss 350 hubs) with the idea that I'd upgrade to 30mm carbon rims when the 28mm aluminum set went bad. Funny thing though, the aluminum wheelset has held up quite well for over a year. I have been running it with Tubolight EVO inserts.

Race Kings top out at 2.2 and there's no finer xc tire.

(and this is coming from someone with 2.5 MaxxGrip DHF's and Cushcore on a Stumpy Evo)

---

I talk a lot - Give it a listen: http://www.fasttalklabs.com/category/fast-talk
I also give Training Advice via http://www.ForeverEndurance.com

The above poster has eschewed traditional employment and is currently undertaking the ill-conceived task of launching his own hardgoods company. Statements are not made on behalf of nor reflective of anything in any manner... unless they're good, then they count.
http://www.AGNCYINNOVATION.com

I didn't like 2.4 tires with 25mm rims. Once I moved to 29mm rims I love 2.4 tires. The wider internal rims seem to support the sidewalls better and makes cornering feel sharper and less vague.

My bike came with 23mm rims and I'm running a 2.6 front and a 2.25 rear.

The manufacturer says the rim is too narrow for a 2.6 (Stans Crest MK3) but it works just fine.

I will move the rear to a 2.6 when the old one wears out. (if it fits in the frame)

I just nuked the rear hub on the OEM Rovals on my trail bike, so went with the Hunt Race XC Wide as the replacement. Will see how that feels. I've only ever used the Rovals being newer to MTB.

I'm on a Canondale Scalpel with a Lefty front end and some weird AI offset rear. I'm not changing any time soon.